1. Field of the Invention
The present invention relates to a heater control system for an air-fuel ratio sensor of an internal combustion engine or, in particular, to a heater control system for an air-fuel ratio sensor of an internal combustion engine for supplying power to a heater in such a manner as to activate the air-fuel ratio sensor quickly when the engine starts.
2. Description of the Related Art
A heater control system for an air-fuel ratio sensor arranged in the exhaust system of an internal combustion engine has been proposed (in JPP No. 3-246461), in which, in order to activate the air-fuel ratio sensor quickly when the engine starts, a high voltage is applied to a heater for heating the air-fuel ratio sensor immediately after starting the engine, in which the time when the air-fuel ratio sensor is activated is detected by the fact that the voltage between two porous electrodes of an electrochemical sensor cell has dropped below a predetermined value, and in which the voltage applied to the heater is subsequently reduced to a low voltage. In a generally-used conventional heater control operation for the air-fuel ratio sensor, a battery voltage is applied to the heater at a predetermined duty cycle, and this duty factor is set to, say, 100% until the air-fuel ratio sensor is activated and subsequently to, say, 50%. This heater control operation will be explained below.
FIG. 17 is a diagram schematically showing a configuration of a heater control system for an air-fuel ratio sensor of an internal combustion engine according to the prior art. FIG. 18 is a time chart of the voltage applied to the heater, the heater temperature and the sensor element temperature of the heater control system shown in FIG. 17. As shown in FIG. 17, a heater 1 is heated by being supplied with power from a power supply including a battery 2 and an alternator 20, and thereby activates the air-fuel ratio sensor (not shown). The current is supplied or cut off to the heater 1 by closing and opening a first switch SW1. SW1 is composed of a FET, for example, and is arranged in a control means 5 for controlling the power supplied to the heater 1. An external terminal T1 of the control means 5 is connected to the battery 2 through an ignition switch IGSW and a fuse F2, an external terminal T2 is grounded, and an external terminal T3 is connected to an end of the heater 1. Terminals BA, ON and ST of the ignition switch IGSW are connected to the positive electrode of the battery 2, the fuse F2 for the external load and the fuse F1 for the heater 1, respectively.
When the ignition switch IGSW is turned on from off state, the voltage across the battery 2 is applied to the heater 1 through the fuse F1. When the control means 5 turns-on the switch SW1 subsequently, a current flows in the heater 1. The current flowing in the heater 1 is detected by a current detection circuit 11 based on the voltage across a current detection resistor r.sub.11 arranged in the control means 5. The voltage applied to the heater 1, on the other hand, is detected by detecting the voltages at the external terminals T1 and T3 and determined from the difference therebetween. The voltage at the external terminal T1, i.e., the voltage across the battery 2 is detected by an electronic control unit (ECU) not shown, while the voltage at the external terminal T3 is detected-by a voltage detection circuit 12. The ECU is composed of a digital computer and includes a ROM (read-only memory), a RAM (random access memory), a backup RAM, a CPU (microprocessor), and input and output ports connected to each other by a bidirectional bus. The FET used as the switch SW1 is turned on and off by a current control circuit 13 connected to the output port of the ECU. Now, the control operation of the current control circuit 13 will be explained.
In FIG. 18, the abscissa represents the time, and the ordinate represents the voltage applied to the heater for a curve V.sub.a, the heater temperature for a curve T.sub.a, and the sensor element temperature for a curve T.sub.b. At time point t.sub.0 when the ignition switch IGSW is turned on, the voltage V.sub.B across the battery 2 is directly applied to the heater 1. At this time, the voltage V.sub.HT applied to the heater 1 is equal to V.sub.B. At time t.sub.1 when the engine is started, i.e., when the ignition switch IGSW is turned to position ST, the ECU starts to drive a starter motor not shown. Thus the voltage V.sub.B across the battery 2 (indicated by the curve V.sub.a) drops sharply. With the rotation of the starter motor, the alternator 20 generates power and begins to charge the battery 2. Consequently, V.sub.B begins to rise gradually, and after passing a predetermined voltage V.sub.TH at time point t.sub.2, reaches the maximum output voltage V.sub.ALT of the alternator 20. In order for the voltage drop across the battery 2 not to adversely affect the startability of the engine, the current begins to be supplied to the heater at time point t.sub.2.
With the rise in the temperature of the heater 1, the sensor element is heated. The heater temperature (indicated by curve T.sub.a) and the sensor element temperature (indicated by curve T.sub.b) rise until time point t.sub.3 when the alternator 20 produces a maximum output voltage. After that, at time point t.sub.4, the sensor element reaches an activation temperature T.sub.th of, say, 650.degree. C., indicating an active state, thus making it possible to measure the air-fuel ratio (A/F). At and after time point t.sub.4, the temperature of the heater 1 is controlled in such a manner as to maintain the active state of the sensor element. The temperature of the heater can be controlled by various methods. They include a method in which the resistance value of the heater is measured and controlled at a constant value, a method in which the power supplied to the heater is controlled based on a power map prepared in accordance with the operating conditions of the engine, and a method in which the resistance value of the sensor element is measured and controlled at a constant value.
A method of controlling the power supplied to the heater will be briefly described below. The sensor element, which is arranged in the exhaust pipe, receives the heat from the exhaust gas as well as from the heater arranged in the exhaust pipe, and further receives the radiation heat from the exhaust pipe and the engine body. As a result, the temperature of the sensor element is affected not only by the temperature of the heater but also by the temperature of the exhaust gas and the temperature of the engine body. In view of this, power is supplied to the heater based on the basic electric energy determined in accordance with the operating conditions of the engine. Specifically, the lower the load under which the engine running, i.e. the lower the exhaust gas temperature, the higher the level to which the basic electric energy of the heater is set. The higher the load under which the engine is running, i.e., the higher the exhaust gas temperature, on the other hand, the lower the level at which the basic electric energy of the heater is set. Also, this basic electric energy is determined experimentally in such a manner as to maintain the temperature of the sensor element in the range of 650.degree. C. to 750.degree. C. in order to maintain the sensor element in active state.
The above-mentioned control method will be specifically explained. The resistance value of the heater 1 is calculated from the current flowing in the heater 1 detected by the current detection circuit 11 and the voltage applied to the heater 1 detected by the voltage detection circuit 12. From the resistance value of the heater 1, the temperature of the heater 1 proportional to the particular resistance value is calculated. Power is then supplied to the heater 1 in such a manner that the temperature of the heater 1 may be maintained at a sufficient level to hold the air-fuel ratio sensor in an active state. Also, the power supplied to the heater 1 is controlled by turning on and off the switch SW1 in a predetermined duty cycle in accordance with the duty factor calculated based on the basic electric energy corresponding to the operating conditions of the engine.
By the way, in the system proposed in JPP No. 3-246461 described above and in a generally-used conventional heater control system for an air-fuel ratio sensor, the voltage applied to the air-fuel ratio sensor has an upper limit thereof determined by the performance of the battery and the alternator mounted on the vehicle carrying the engine. In rapidly heating the air-fuel ratio sensor, the restriction imposed by the particular upper limit poses the problem that the air-fuel ratio sensor cannot be activated at a sufficiently early time.
Another possible method for rapidly heating the air-fuel ratio sensor is increasing the current by reducing the resistance value of the heater while maintaining constant the voltage supplied to the heater. The problem hampering the realization of this method is the fact that the increase in heater size and the improvement in the material have their own limits and lead to a high cost.
Also, in these conventional systems, the supply of current to the heater is inhibited during the cranking of the engine in order to secure the starter performance, i.e., the startability of the engine. This gives rise to the problem that the activation of the air-fuel sensor is further delayed.